Retinal cameras that transfer digital camera images using digital transmission between the camera and the image-rendering system have been employed in the past to take the 6- to 11-megapixel digital camera outputs and transfer the data over parallel lines in a cable to follow-on processing. Thus, the images from retinal cameras are transmitted to either a local processor over for instance 10 feet, or around the world so that the digital images may be remotely displayed.
There has however been a problem with haze due to the dark currents induced by circuits housed within the camera. The current draw of these circuits causes the CCD sensor array to heat up in a matter of minutes. This results in images containing haze because the sensor array in the camera has difficulty distinguishing between heat and light. It will be appreciated that haze obscures the detail in the retinal image because it ruins contrast. Instead of images appearing black, nothing in the image looks black but rather gray. Thus the thermally induced haze deadens contrast.
It will be appreciated that retinal cameras have utilized analog systems for transferring the data from the digital camera array to a remote location.
However, if the information is transferred in analog form, the transmission is oftentimes corrupted by outside noise. It will be appreciated that the CCD sensor pixels have output voltages proportional to the incident light. Thus, for each pixel, one obtains a voltage change that can be measured. In the analog domain, one takes the voltage information and amplifies it before coupling it by a cable to a computer. It is noted that these analog voltages are directly transmitted and are not analog-to-digital converted into digital equivalents at the camera.
The problems with such analog systems are that, as the signals go down the cable, they are susceptible to noise, whether it be magnetic, electromagnetic, fluorescent light noise or even power line noise. Oftentimes the analog signals can be corrupted by cross talk in which the lines in the cable are wrapped around the same bundle that is generating the noise. By amplifying the signals in these cables to permit long distance signaling the noise is amplified.
Such an analog system is manufactured by Megavision and although the camera itself is in a smaller package than those employing digital data transfer, the digital transfer of image information along a cable is much less susceptible to the aforementioned noise sources.
In order to get away from the analog transmission of data, digital cameras today convert the sensor outputs within the camera to, in one embodiment, 12 bits of data that is transmitted in parallel using differential line drivers and twisted pairs so that for 12 bits of information, one drives 24 wires.
By operating in the digital domain one achieves clean signals that can be sent thousands of feet without corruption.
While digital data transfer is preferable from the noise corruption point of view, it is much more complicated than analog systems. This is because more circuitry is required, which draws more power. Thus the circuits utilized in the analog-to-digital conversion and the line driving consume a fair amount of power that generates heat within the camera. Moreover, when utilizing digital communications streams, the communication protocols are much more complicated in the analog protocols. If one wants to send an analog voltage to a remote location, all one needs is a ground and the voltage. However, in sending digital data, for instance in a 12-bit system, there must be at least 12 lines. Note in 12-bit transfer the sensor pixel value can be anywhere from 0 to 4,095.
The digital information is clocked down the cable, in one embodiment at 14 MHz, which is the clock frequency normally employed. Thus the data rate or frequency associated with such retinal image data transmission systems is relatively high.
It might be thought that one could use serial data and transmit the serialized data down a coaxial cable. However, serial connections are relatively slow, especially when considering that one picture contains as many as 6 to 11 megapixels that must be transmitted down the line every second. Thus even with serial methods such as firewire, the amount of data transmitted is limited. When utilizing a parallel system in which each of the 12 bits is dedicated to a wire pair, the transmission is much faster due to the parallel interface.
Note, in a 12-bit system the differential line drivers employed in the camera drive 24 lines, with at least four other hand-shaking signals adding another 8 lines.
While the cause of dark current was assumed to be the current draw of the circuits used within the digital camera, it was not immediately clear what the source of this current was. Analysis has shown that he majority of the current draw is from the line drivers and the clock driver. Thus while dark current is not much of an issue with analog cameras, the heat associated with the current draw in digital transmission systems changes the properties of the CCD sensor in a matter of minutes.
After analysis of the power consumption of the retinal camera circuits, it was found that the drivers were pulling as much as 2 amps, which significantly degrades the retinal camera images such that after a minute or two one is seeing haze. As mentioned hereinbefore, if the image is hazy, detail is obscured because of the lack of contrast between the dark areas and the light areas in the image. As will be appreciated, when looking at an image corrupted by haze, nothing black appears black but rather gray.